Graphics rendering systems create images of objects which are combined in a visual scene. An object is a computer readable specification of appearance attributes which, when used to create an image, has the appearance of physical substance. A scene is a collection of objects distributed around an area to be represented in an image. In a two dimensional graphics rendering system an image is constructed based on the location and orientation of two dimensional objects in a scene. For a three dimensional graphics rendering system, three dimensional objects are placed in a three dimensional scene with a three dimensional coordinate system. A camera is defined by at least a location and a direction of view relative to a scene. Rendering is the process of creating an image based on the objects which would be visible to a camera viewing a scene if it were real, and placing this image in memory, typically a frame buffer. The image is composed of an array of picture elements, or pixels, which each exhibit a color. In real-time rendering systems, the image is displayed, typically on a computer monitor, while a later image is being constructed. The part of the rendering system which interprets object data to determine what the scene looks like is referred to as the rendering pipeline.
High speed rendering systems typically rely on combinations of simple polygons, referred to as primitives, to build more complex objects. The rendering pipeline of such a system is generally optimized to render primitives into the frame buffer quickly. Triangles are commonly used as primitives, since objects of arbitrary complexity may be composed of triangles. This is illustrated in FIG. 1.
The discrete pixels of an image in a frame buffer are comparable to samples of a continuous image. A well known phenomenon associated with discrete sampling of continuous values is aliasing. In the field of computer graphics rendering, aliasing is most often encountered in the form of straight lines which have a jagged or stair-stepped appearance, as illustrated in FIG. 2. The edges of primitives (such as triangles) rendered to an image may exhibit this pattern, which is especially noticeable where there is high contrast between the color of a foreground primitive and the color of the background. This aliasing of primitive edges is generally undesirable, and steps are taken to reduce the effect of it.
If nothing is done to reduce the effects of aliasing, a pixel which represents an area of a scene containing an edge of high color contrast in a computer generated image will generally be colored according to whichever color happens to coincide with the centroid of the pixel. This is illustrated in FIG. 3, where a pixel is shown representing an area which is partly red and partly blue. The pixel is given the color red, because the centroid of the pixel falls on the red primitive. A more realistic image, and one without noticeable aliasing effects, would be obtained if the pixel were colored with both red and blue, in the proportion each is present in the area represented by the pixel. This blending of colors is at the heart of most schemes to reduce the effects of aliasing. Efforts to reduce aliasing in the field of computer graphics are referred to as anti-aliasing.
One method of performing anti-aliasing, known in the art as sub-sampling, is to determine colors for a number of samples within the area represented by each pixel. Each of these sub-samples is at a slightly different location, and the sub-samples are averaged together to determine a final color for the pixel. This method reduces aliasing considerably, but at the expense of increasing the amount of calculation, and time, required for rendering each pixel. The time expense is so large that this solution is not generally used for real-time rendering systems.
A solution which is feasible for real-time rendering is to blend the color of each pixel in an image with the colors of surrounding pixels. This is, in effect, a low-pass filter applied to the initial image determined by the rendering pipeline. The added amount of calculation is much less than for the sub-sampling solution, but the results are poor. The entire image is blurred, and appears to be out of focus.
Another solution to the problem of anti-aliasing real-time computer generated images is to only apply anti-aliasing techniques to areas of an image which correspond to object silhouette edges. A silhouette edge is the visible perimeter of an object. Sharp contrasts (and therefore areas of noticeable aliasing) are generally most likely to occur at silhouette edges. Finding the portions of an image which correspond to object silhouette edges is not trivial. One method of finding these edges is to use a buffer which holds one bit per pixel of the finished image. The buffer is set to all zeros, then as each object is rendered the state of the bits in the buffer corresponding to the drawn pixels are changed. When all objects have been rendered, the bits of this buffer will have gradients from one to zero or from zero to one in areas corresponding to the silhouette edges of many of the objects. The corresponding areas in the image are then subjected to low-pass filtering. This method, however, uses a lot of memory for the buffer, does not always catch all of the object silhouette edges and generates a lot of false edges.
What is needed is a system and method for performing anti-aliasing on those parts of a rendered image which should be anti-aliased, without disturbing those portions of the image which should not be anti-aliased. To do this a system should accurately determine object silhouette edges without requiring intensive additional computing or large amounts of additional memory.